This invention relates in general to machines used to measure torque and power, such as dynamometers, and in particular to dynamometers having a chassis to chassis load measurement device.
Dynamometers have been used for many years in the vehicle manufacturing industry for testing vehicles in place in addition to, or in lieu of, on-road testing. Known vehicle dynamometers may be used in a variety of testing operations. In one known application, a dynamometer is used to measure the torque and horsepower output of a vehicle. In another known application, a dynamometer is used to simulate the road load forces and the inertia forces that act on a vehicle during vehicle operation on a roadway. In both applications, users require that the dynamometer be capable of measuring roll torque with a high degree of accuracy.
One known chassis roll dynamometer comprises at least one dynamometer roller that is rotatably mounted in a stationary frame. The roller is configured to support at least one driven wheel of a vehicle to be tested. In some embodiments the roller may have a large diameter and may be several times larger than the diameter of the supported vehicle wheel. In other embodiments, the roller may have a diameter smaller than the diameter of the supported vehicle wheel. There is a driving engagement between the vehicle wheel and the roller due to the traction forces of the wheel, such that the vehicle wheel may drive the roller. Additionally, in some types of dynamometers the roller may drive the vehicle wheel.
Known dynamometers also include a device for developing a braking torque between the roller and dynamometer frame. Many types of devices for developing the braking torque are known and include frictional and hydraulic devices. Additionally, electromagnetic devices, such as a motor/generator brake and an eddy current brake are known. Known dynamometers also include an apparatus for simulating road load forces acting on the vehicle wheel, and/or for simulating inertial forces acting on the vehicle during acceleration and deceleration. Such road load forces include rolling friction and windage. The apparatus for simulating road load forces may comprise a motor coupled with the roller through a roller shaft and a controller for controlling energization of the motor in accordance with the simulation or test being conducted. In some known dynamometers, a flywheel is coupled with the roller shaft for simulating inertia. The motor in known dynamometers is often referred to as a power exchange unit because it may be operated to either apply power to the vehicle wheel, or to absorb power from the vehicle wheel through the roller. Additionally, the motor may operated as a torque generating device, or as a torque absorbing device and may be either a DC motor or an AC motor.
In known dynamometers, such as those described above, it is common practice to provide instrumentation for the measurement of the torque output and rotational speed of the vehicle wheel. The torque output is typically measured by a torque transducer connected in the drive train of the dynamometer with the transducer providing a roll torque signal to an electronic dynamometer controller. Wheel speed, which is generally the same as roller speed, is typically measured by a shaft encoder suitably connected to the dynamometer, and which supplies a rotational wheel speed signal to the dynamometer controller. The dynamometer controller typically includes a computer and monitors the dynamometer torque output and rotational wheel speed while controlling the applied load torque.
Known methods of measuring force generated by a vehicle using a conventional chassis roll dynamometer include: (1) calculating force by measuring the time it takes to rotationally accelerate a drum having a known mass. (2) physically measuring torque from the dynamometer shaft though a torque arm attached to either an electric, a mechanical, or a hydraulic brake, and (3) combining the data from methods 1 and 2 to provide an average or median of the data generated by the two methods. These common methods of measuring force with a conventional chassis roll dynamometer have the disadvantage that they cannot accurately account for all of the drag losses in the conventional chassis roll dynamometer system. Thus, it would be desirable to provide an improved dynamometer that minimizes such drag losses, and thereby improves the accuracy of a force measurement.